Until 31 March 2009, National Rail is offering the one year Family & Friends Railcard at just £18. These cut a third off an adult's ticket and 60% off kid's tickets when you have at least one adult and one child travelling. You can have up to four adults and four kids travel on one card.
It's only available online. You'll need to enter the special code EXPERT25 when registering.
The Railcard can't be used on weekday morning peak services entirely within the London and South East area. If you're unsure when these times are, as they differ according to the route, then check with your local station or call the National Rail Enquiries line on 08457 48 49 50.
On previous occasions this offer's been on some people waiting until the last day to register have experienced problems so it's advisable to do it at least a few days before it ends.